It is known to arrange auxiliary brakes in a vehicle as a supplement to the service brakes of the vehicle. Auxiliary brakes are used mainly in heavy-duty vehicles, such as trucks, buses and construction machinery for the primary purpose of sparing the service brakes of the vehicle. This is especially true when driving on long downhill gradients when it is important that the vehicle does not develop too high a speed. By making use of the auxiliary brakes, the service brakes can be preserved so that, when the vehicle really has to decelerate very strongly, they can deliver maximum braking force. The service brakes have a much more powerful braking effect than auxiliary brakes, partly due to the fact that the service brakes are normally arranged on all the wheels on the vehicle. The various types of auxiliary brake are arranged so as only to act on the driving wheels of the vehicle via the drive line of the vehicle. The terminology of “drive line” shall be used herein to mean the engine, transmission including clutch and gearbox, and final gear of the vehicle, as well as other components out to the driving wheels.
It is also known to differentiate between what are known as primary and secondary auxiliary brakes in a vehicle. Primary and secondary refer to the positioning of the auxiliary brake before or after the main gearbox of the vehicle and its clutch device. Examples of primary auxiliary brakes are ISGs (Integrated Starters and Generators) and retarders. A retarder is usually of the hydrodynamic or electromagnetic type. These are arranged between the engine and the main gearbox.
A primary auxiliary brake can also consist of various types of engine brake, for example a compression brake or exhaust-gas brake. The braking energy in a compression brake and an exhaust-gas brake is converted mainly to heat, which to a great extent is dissipated via the cooling system of the engine, but it should be noted that a considerable part (roughly 40% of the braking energy) leaves the vehicle via the exhaust pipe through the gas exchange of the engine.
A secondary auxiliary brake, which is arranged somewhere after the main gearbox of the vehicle and its clutch device, usually consists of a retarder of hydrodynamic or electromagnetic type. As the secondary auxiliary brake is arranged after the clutch device of the vehicle, it can brake even when the clutch is disengaged or when the gearbox is in neutral position.
As the primary auxiliary brake is arranged before the clutch device of the vehicle, it cannot brake when the clutch is disengaged; that is to say, when the clutch is disconnected, so that no torque is transmitted between engine and transmission. The result of this is that a primary auxiliary brake can brake the vehicle only when the clutch is engaged; that is to say, only when it transmits a braking torque from the auxiliary brake to the driving wheels. This means that the braking effect of the auxiliary ceases when disengagement takes place, for example in connection with shifting. This can cause problems on a steep downhill gradient, for example, in a case where the vehicle is equipped with a semi-automatic gearbox; that is to say, an automatically shifted manual gearbox. These gearboxes are often unsynchronized. When the downshift starts, the gearbox is disengaged which leads to the braking torque of the primary auxiliary brake being lost. There is then a risk that it will be possible for the speed of the vehicle to increase to such an extent that the lower gear can no longer be engaged.